Generally speaking, there are known vehicle crash detection systems which attempt to detect side impacts using accelerometers mounted on the B-pillar, the rocker panel, the reinforcing beam, or some other location on or near the door of the vehicle. Other known systems use contact switches, or sense pressure changes within the door. Some distributed sensing systems have also been proposed.
These systems have not proven entirely satisfactory. More specifically, signals received by accelerometers and contact switches vary greatly depending on the location, magnitude and direction of impact. These arrangements only provide localized acceleration and velocity information, which does not necessarily reflect what is happening away from their mounting point. For example, a 15 mph pole impact (a typical airbag deployment scenario) which occurs at the sensor mounting location can induce a relatively large amplitude signal which initiates a proper air bag deployment, while one that occurs a foot or two away may give sensor signals having significantly lower amplitude thereby resulting in non-deployment of the air bag. This makes it very difficult to discriminate "fire" conditions from "no fire" situations in many cases.
Distributed sensor arrangements are difficult to install and maintain. Their cost may also be excessive due to the need for increased wiring and mounting requirements.
Furthermore, most accelerometers require a power source and thus present another potential point of system failure. Yet further, post-installation calibration of such sensors must generally be done electronically. While this may insure accurate sensing of acceleration relative to the module, it can not account for changes in the module mounting support or orientation which could result from damage caused by prior "no fire" collisions.
U.S. Pat. No. 5,580,084 discloses and claims a system and method for controlling actuation of a vehicle safety device by detecting variations in the magnetic field that is influenced by a ferromagnetic element mechanically coupled to a portion of the vehicle subject to plastic strain, whereby the plastic strain creates elastic strain waves in the ferromagnetic element which causes the ferromagnetic properties thereof to be altered by the strain waves though the process of magnetostriction. U.S. Pat. No. 5,580,084 does not teach the generation of a signal from the change in reluctance of a magnetic circuit caused by the rigid body motion of the elements therein. Furthermore, the variations in the magnetic field caused by elastic strain waves in the ferromagnetic element resulting from magnetostriction provides in itself a very low amplitude signal which is relatively insignificant with respect to variations in magnetic field caused by relative motion of rigid body elements in an associated magnetic circuit resulting from macroscopic magnetic inductive effects.